This invention relates to a method of recovering gold from gold bearing chlorination leach solutions.
Electrolytic refinery slimes have high concentrations of precious metal such as gold and silver and it is required to recover these precious metals in separate form from the slimes. One of the processes in commercial operation for gold recovery is described in J. E. Hoffmann, "Hydrometallurgical Processing of Kennecott Refinery Slimes" in the Proceedings of COPPER 95-COBRE 95 International Conference, Volume III--Electrorefining and Hydrometallurgy of Copper, The Metallurgical Society of CIM 1995. The process starts with the gold bearing leach solution obtained by chlorination leaching of decopperized slimes and comprises the steps of (1) extracting gold from the leach solution with dibutyl caribtol (DBC) used as an organic extractant, (2) scrubbing the resulting gold bearing organic phase with hydrochloric acid and (3) reducing gold in the organic phase with an added aqueous solution of a reducing reagent. Dibutyl carbitol which is not only stable but also low in volatility and toxicity is a suitable solvent for use in solvent extraction of gold from chloride bearing aqueous solutions. Because the distribution coefficient of gold between the organic phase of DBC and the aqueous phase is as high as about 1000-3000, it is anticipated that 99 wt % of the gold will be transferred into the organic phase if two-stage extraction with DBC is performed. The concentration of hydrochloric acid in the gold bearing leach solution is about 3 N because the concentration will not only provide high efficiency in chlorination leaching but also ensure the acuptable low level extraction of impurities into dibutyl carbitol. The gold bearing organic phase obtained in step (1) is scrubbed with HCL in step (2) because selenium, tellurium, arsenic, antimony and bismuth have been extracted together with gold. The scrubbing is usually performed through multiple stages in a countercurrent fashion on a batch or continuous basis employing 1-1.5 N HCl. Because of its high distribution coefficient between the DBC phase and the aqueous phase, gold is not stripped but reduced directly from the organic phase by contact with an aqueous solution of a reducing agent so as to produce metallic gold. Oxalic acid is used in most cases as the best reducing agent capable of recovering gold in high purity.
The purity of the gold recovered by the process described above is only about 99.997-99.998 wt % at maximum and needs to be further increased in purity.